Conventional fingerprint imaging systems may implement distortion correction, also known as rectification, to correct for distortion that may typically include correction of radial distortions introduced by lenses. Such radial distortions may include barrel and pincushion distortion. Rectification also may correct for perspective distortions—those distortions which may be introduced into an imaging system due to the physical relationship between a principle point on the lens (e.g., the iris) and the scene being captured. Some fingerprint imaging systems may rectify radial and perspective correction in the system. For example, some conventional systems include hardware that may rectify radial and perspective distortion. However, other distortions may be present within a captured image due to curved surfaces in the light path. For instance, imaging through glass balls or from the reflection in a hyperbolic mirror may introduce distortions in a captured image.
Collectively, these types of image distortions may be referred to as systematic imaging distortions. More specifically, systematic image distortions may include those distortions caused by surfaces within a fingerprint imaging system at which light may be processed (e.g., lens surfaces, mirror surfaces, other refractive surfaces, other reflective surfaces, and/or other surfaces formed by optical elements).
Generally, conventional rectifying solutions share a focus on correcting images that have been captured with a simple optical configuration within a fingerprint imaging system—a system which may include a camera and a lens that takes images or photographs of a scene in three dimensions. Conventional solutions often fail to address more complex optical configurations that include optical elements beyond the lens. That is, the simple imaging systems may not compensate for images captured with multiple optical elements, or a more complex optical element, in the light path, such as windows, prisms, non-aligned lenses, balls, or other transparent or semi-transparent media or reflective surfaces.
Traditionally, attempts to minimize distortion in fingerprint imaging systems have been conducted during a design phase. Often, an iterative design process may be used in which building and analyzing prototype systems results in the final fingerprint imaging system design. The resulting design that meets the distortion goals may incorporate allowable tolerances on system components. Often, additional optical components may be required in the design to correct distortions to an acceptable level.
During manufacturing, fingerprint imaging systems may be built according to the final fingerprint imaging system design. Typically, components are assembled and during a quality assurance step, the components are adjusted to ensure that the distortions in each manufactured system do not exceed various tolerances. In order ensure that system distortions do not exceed these tolerances, optical components of a relatively high quality and expense are often used. For example, to adhere to the overall system tolerances, the components themselves may be required to be manufactured with high precision, thereby increasing a cost of manufacturing the corresponding optical component.
In some instances a calibration may be implemented to assure that the optical components are aligned with a predetermined precision (i.e., within predetermined alignment tolerances). Calibration typically involves adjusting component locations with respect to one another to correct distortion in images captured by the manufactured systems. This calibration step can be rather involved for higher precision fingerprint imaging system designs with relatively tight alignment tolerances.
During operation, performance of a fingerprint imaging system may degrade over time. Generally, restoring the calibration of a fingerprint imaging system typically includes a physical adjustment of components within the system to correct for image distortions, and to ensure that the alignment of the optical components within the fingerprint imaging system falls within the predetermined alignment tolerances, which may be costly and/or time consuming. Other drawbacks in conventional fingerprint imaging systems exist.